1. Field of the Invention
This invention relates to the use of an improved attrition resistant catalyst in oxidation processes wherein the oxidation is performed by an oxidized catalyst and the reduced catalyst is separately regenerated, and wherein the attrition resistant catalyst is made by the steps of forming a slurry containing a material with catalytic properties, spray drying the slurry, and calcining the spray dried product.
2. Description of the Related Art
Preparation of attrition resistant catalysts containing a binder ingredient is known in the art. In one method, the catalyst is made by the steps of forming a slurry containing a material with catalytic properties, spray drying the slurry, and calcining and activating the resulting spray dried microspheres. The binder ingredient is any inert support or binder such as silica, alumina or aluminum silicate. Besides spray drying, other techniques are known for preparing attrition resistant catalysts such as impregnation of the catalyst or its precursor on porous spherical support particles.
In particular, certain prior art procedures have involved combining a silica component in the form of a solution or colloidal sol, generally at a concentration of SiO.sub.2 above 10 wt % to as high as 50 wt %, with a catalyst, catalyst precursor, or catalyst support to form a slurry. The slurry is then spray dried, calcined and activated to form the catalyst. The silica component, which functions as a binder, is normally dispersed throughout the particles.
U.S. Pat. No. 4,677,084 to Bergna discloses an improvement in the process of preparing attrition resistant catalysts, in which a solution of oxide precursor is used to form an oxide shell over particles of catalyst, catalyst precursor or catalyst support. The process includes the steps of forming a slurry of the catalyst, catalyst precursor or catalyst support particles with a solute of the oxide precursor having a particle size of not greater than 5 nm, spray drying, and calcining. The amount of solute and particles are selected such that the oxide shell which forms is comprised of 3 to 15% by weight of the total weight of particles and oxide. Aqueous silicic acid, i.e. polysilicic acid, with a SiO.sub.2 concentration below 6 wt % is preferred as the oxide precursor. Examples show that satisfactory attrition resistance is achieved with much lower SiO.sub.2 content in the dried catalyst than prior art catalysts.
The attrition resistance of the above catalyst has proven to be excellent in typical vapor phase catalytic processes where the catalyst is used in a single valence state, that is, where the catalyst is continually regenerated in the reaction vessel by the incoming reactants and is essentially unchanged by the reaction.
However, in certain vapor phase catalytic oxidation processes it is advantageous to instead carry out the reaction in two steps, in the first of which the oxidized form of a catalyst is used to oxidize the reactant, and in the second of which the reduced catalyst from the first step is separately regenerated. Such two-step vapor phase processes can be conveniently carried out in recirculating solids reactors, also referred to in the literature as transport bed reactors or circulating fluidized bed reactors, as well as by other names. The literature discloses such processes have been found advantageous for the oxidation of butane to maleic anhydride, the oxidation of propane to acrolein and acrylic acid, the ammoxidation of propylene to acrylonitrile, and various other applications. When used in this type of process, we have found that the attrition resistance of the catalyst made by the Bergna patent can be inadequate in certain cases, although adequate in other cases.
A variation of the catalyst preparation used in the Bergna patent is disclosed in U.S. Pat. No. 5,302,566 to Schwartz, aimed at increasing the stability against gelling of the freshly made polysilicic acid used in the above process. This patent discloses a modified process in which the gelling problem is corrected by adding to the catalyst/polysilicic acid slurry a colloidal oxide sol having oxide particles averaging in size between 5 and 7 nm, wherein the amount of colloidal oxide sol provides between 50% to 95% by weight of the oxide shell and the polysilicic acid provides 50% to 5%, and wherein the oxide shell is between 3% and 15% by weight of the total weight of catalyst particles and shell. Under the conditions tested, as shown in the patent's FIG. 1, this modified catalyst was equal or slightly inferior in attrition resistance to the Bergna patent catalyst. It was not tested in the previously described type of two-stage oxidation system.
There is a need for a catalyst with improved attrition resistance for certain two-step oxidation processes wherein the oxidation is performed by an oxidized catalyst and the reduced catalyst is separately regenerated.